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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

BENZOLES

325

The separation of the three xyloles (xylenes), which is usually not required
cannot be effected by means of fractional distillation, their boiling points being
too similar. The respective quantities may, however, be determined approxi-
mately by regarding the distillate between 135° and 137° as £-xylene, that
between 137° and 140° as m-xylene, and that between 140° and 145° as o-xylene •
these limits refer to uncorrected temperatures, i.e., those indicated by "a ther-
mometer with its scale only partially immersed in the vapour.

4. Detection and Estimation of Impurities.—The impurities of
commercial benzoles are principally carbon disulphide, thiophene, paraffin
hydrocarbons and naphthalene.

(a) CARBON BISULPHIDE. This is detected by shaking about 10 c.c. of
the benzole or, better, of the first fractions of its distillate, with 5 or 6 drops
of phenylhydrazine and leaving the mixture at rest for an hour. In presence
of as little as 0-2% of carbon disulphide, a white precipitate of phenylhydra-
zine phenylsulphocarbazinate is formed.

For the determination, the ammonium xanthate (Hoffmann) reaction is
used:

A mixture of 50 grams of the benzole with 50 grams of alcoholic potash
solution (n grams of KOH in 90 grams of absolute alcohol) is left for some
hours at the ordinary temperature and is then shaken with 100 c.c. of water.
The aqueous layer is separated from the benzole, which is washed two or
three times with water, the total aqueous liquid being made up to 400 c.c.
In this solution, or an aliquot part of it, the potassium xanthate formed is
determined volumetricalry by means of a standard copper solution (12-468
grams of crystallised copper sulphate per litre).

This is effected by acidifying the aqueous liquid containing the xanthate
with acetic acid and then adding the copper sulphate solution until the
copper is in excess, i.e., until a drop of the liquid gives the brown coloration
with potassium ferrocyanide. The number of c.c. used, multiplied by
0-0076, gives the percentage of CS2 in the aqueous liquid and from this the
percentage in the benzole may be calculated.

(V) THIOPHENE. This is detected by the indophenine reaction. To a
few granules of isatin in a porcelain basin, a few c.c. of pure cone, sulphuric
acid are added and then the benzole, the liquid being covered with a clock-
glass and left to itself for some hours : in presence of thiophene, blue rings
form around the isatin granules.

Only benzoles guaranteed free from thiophene are tested for the latter.

(c) PARAFFIN HYDROCARBONS (benzines). These are determined by
transforming the benzoles into the soluble sulpho-acids (Kraemer and Spil-
ker)1: 200 grams of the benzole are shaken for 15 minutes in a separating
funnel with 500 grams of fuming sulphuric acid (20% S03), cooled if necessary
and left for two hours. The sulphuric acid is removed and the operation
repeated twice. The residual unattacked oil floating in the sulphuric acid
represents almost the whole of the paraffin hydrocarbons (including naph-
thenes) contained in the 200 grams of benzole.

1 Muspratt:   Chemie, 4th edit., Vol. VIII, p. 34.egards the limits of